1. Field of the Invention
The present invention relates to a method for inspecting a bonded status of a ball bonded to a pad of a semiconductor chip by image processing in wire bonding which attains wiring between the pad of the semiconductor chip and an external electrode.
2. Description of the Related Art
Referring first to FIGS. 2A and 2B, a bonded condition between a pad of a semiconductor chip and a ball in wire bonding will be described.
In FIGS. 2A and 2B, reference numeral 1 denotes a lead frame; 2 denotes a semiconductor chip mounted on the lead frame 1; 3 denotes a pad formed on the semiconductor chip 2 (a first bonding point); 4 denotes a bonding wire such as a gold wire; and 5 denotes a ball formed at the tip of the bonding wire 4 and bonded to the pad 3.
The ball 5 formed at the tip of the bonding wire 4 has an almost spherical shape before bonding and the ball 5 is pressedly bonded onto the pad 3 by applying a supersonic vibration to the ball 5 while pressing it with a capillary 6 at the time of wire bonding, as shown in FIG. 2A.
At the center of the top surface of the ball 5 bonded to the pad 3, a cone portion 8 with a conical shape is formed by an inside chamfer portion 7 recessed into the inside of the tip of the capillary 6, and an almost flat upper surface 9 is formed around the cone portion 8 of the ball 5.
Conventionally, when quality of bonding is checked by detecting a thickness t, an external diameter D, a bonded point (center position) O of the ball 5 bonded to the pad 3 and the like, each dimension is measured based on vertical and/or horizontal displacements which are obtained by the use of a metallographic microscope and the like which can measure vertical and horizontal displacements by an operator manually focusing the microscope on target locations. However, In the case of such a manual measuring method, it is difficult to obtain a sufficient measurement accuracy because variations are generated depending on the operator who judges whether the microscope is accurately focused on the target locations.
In order to solve such a problem, a method is proposed to measure the thickness t of the bonded ball 5, wherein a plane image of the cone portion 8 of the ball 5 captured through a lens of a camera is compared and checked with a reference pattern image of the cone portion which is set in advance and an image taking height position at which the images are best matched is regarded as an in-focus height (Japanese Patent Application Laid-Open Publication No. 6-224267).
However, in the case of conventional inspection methods for the bonding portion between the pad and ball utilizing image processing including Japanese Patent Application Laid-Open Publication No. 6-224267, the bonding portion between the pad and ball to be inspected is only illuminated by a single type of illumination light such as white light, so that it may be difficult to obtain a clear image with high contrast depending on measurement conditions.
In another inspection method proposed in Japanese Patent Application Laid-Open Publication No. 10-311713, an image of a bonding wire portion connecting between the pad and a lead (external electrode) is taken by a color camera and the obtained color image is converted into each of hue (H), saturation (S) and intensity (I) data, whereby the quality of the wire portion is determined by the use of the HSI data. In an inspection method proposed in Japanese Patent Application Laid-Open Publication No. 7-37955, quality of a wire is determined by obtaining each of RGB images of a wire portion while illuminating the wire portion with ring illumination or laser slit light of red (R) light, green (G) light and blue (B) light and by performing digitalization of each of the obtained RGB images.
However, both of the inspection methods disclosed in Japanese Patent Application Laid-Open Publication Nos. 10-311713 and 7-37955 are intended to inspect the quality of the wire connecting the pad and the lead and cannot be directly applied to the inspection of the bonding portion between the pad and ball to which the present invention is directed.